A lithium ion secondary battery provides the highest energy density among practical batteries, and in particular is often used for compact electronics. Further, the lithium ion secondary battery is expected to be developed for use in automobiles, and expected to have improved output characteristics and to operate stably over a wide temperature range.
In a general positive electrode of such a lithium ion secondary battery, an electrode active material layer is formed on a collector by binding thereto a lithium-containing metal oxide such as LiCoO2, LiMn2O4, or LiFePO4, which is used as an electrode active material for the positive electrode (hereinafter referred to as “positive electrode active material”) with a binder (sometimes referred to as a binding agent) such as polyvinylidene fluoride. On the other hand, in a negative electrode, an electrode active material layer is formed on a collector by binding thereto a carbonaceous (amorphous) carbon material, a metal oxide, a metal sulfide or the like, which is used as an electrode active material for the negative electrode (hereinafter referred to as “negative electrode active material”) with a binder such as a styrene-butadiene copolymer.
In order to solve the problems of output characteristics of the lithium ion secondary battery, attempts have been made to improve the dispersibility of an electrode active material or a conducting agent (also referred to as “conductivity imparting agent” or “conductivity imparting material”) in the electrode.
For example, Patent Document 1 discloses that use of a polymer having a cationic group and an anion corresponding to the cationic group as a binder can improve the dispersibility of an electrode active material, and as a result, the dispersibility of a conducting agent is improved, thereby realizing a battery excellent in electrode surface smoothness and output characteristics.
Further, Patent Document 2 describes that use of a binder containing a polymer having an anionic functional group and a compound having an anionic functional group and a cationic functional group can improve both adhesion with a collector and mobility of lithium ions in the proximity of the surface of an electrode.
On the other hand, in order to solve the problems with low-temperature characteristics of the lithium ion secondary battery, attempts have been made to improve the binding power of the electrode. Patent Document 3 discloses an electrode formed with use of a graphite material as an active material for a negative electrode, together with use of polyethylene or an ethylene-vinyl acetate copolymer as a binder. Patent Document 3 further discloses that heat treatment of the electrode at a temperature equal to or higher than the melting point of the binder results in improvement in binding power in the electrode.